We demonstrated previously that mammalian sex determination requires both GATA4 and FOG2 transcription regulators and relies on their direct interaction to assemble the functioning testis. Our recent data reveals that in gonadal development GATA4-FOG2 transcription complex acts to control the extracellular signaling in both sexes. This application describes the experimental plan designed to characterize the impact of the WNT/DKK signaling on sex determination and development of the female reproductive system. We have identified the Dkk1 gene as a target of GATA4-FOG2 repression in the developing ovary. The expression of Dkk1 was dramatically increased (~10 fold) in embryonic gonads with GATA4-FOG2 interaction loss. We also identified other genes that are no longer expressed or strongly down-regulated in the GATA4 or FOG2 (GATA4/FOG2) XX mutants. Several of these genes (e.g., Wnt4, follistatin (Fst) and Foxl2) are known to be required for normal female development, while other genes (e.g., Sp5, Sprr2d, Irx3 and Gng13) have been recently identified as sexually dimorphic by us and others. We also show that the tissue-specific ablation of the ?-catenin gene in the gonads disrupts female development while testis development in the absence of ?-catenin proceeds as normal. Control of ovarian development as well as the canonical Wnt/??-catenin pathway by the GATA4-FOG2 transcriptional complex presents a novel insight into the crosstalk of transcriptional regulation and extracellular signaling in ovarian development. Now, using available to us 1) Gata4floxed , 2) Fog2floxed and 3) Gata4floxed/ki mice we will delete Gata4 and Fog2 genes during embryogenesis with Sf1Cre to generate fetuses and neonatal animals with Gata4 gene loss, Fog2 gene loss and GATA4-FOG2 interaction loss, respectively. To determine the function for canonical ?-catenin signaling in ovarian development we will analyze mice with gonadal loss of ?-catenin and mice with a loss of the Lrp6 receptor required for canonical ?-catenin signaling. Finally, we will examine sexual development in a) Dkk1 null embryos and b) compound mutant Fog2-/-Dkk1-/-, GATA4ki/ki/Dkk1-/- and Wt1-Sox9/Dkk-/- embryos; and using the tetO-Dkk1 mouse strain we will over-express Dkk1 specifically in embryonic gonads to separate this event from other consequences of the Gata4/Fog2 loss. Analysis of these mouse mutants should provide a novel insight into the fundamental aspects of gonadal differentiation, somatic-germ cell interaction and germ cell differentiation.